Role of Sandstone Mineral Compositions and Rock Quality on the Performance of Low-Salinity Waterflooding

Abstract

Abstract Recent field applications and laboratory studies have recognized that low-salinity waterflooding as a potentially effective technique to achieve sufficient recovery in sandstone reservoirs. It was found that the impact of clay content, rock permeability, and rock quality are still questionable on the performance of low-salinity waterflooding. A set of comprehensive coreflood tests have been conducted to estimate displacement efficiency and investigate the effect of clay content and rock quality using Bandera, Parker, Grey Berea, and Buff Berea sandstone cores. The coreflood experiments have been conducted on 20 and 6 in. length and 1.5 in. diameter outcrop cores at 185°F and 500 psi. Oil recovery, pressure drop, and pH were observed and analyzed after each coreflooding experiment. The mineralogy of the samples was assessed by X-ray powder diffraction, scanning electron microscopy, and X-ray fluorescence. The oil recovery from conventional waterflooding ranged from 24.6 to 44.7% OOIP. The oil recovery decreased when the reservoir permeability decreased. The Bandera, Parker, Grey Berea, and Buff Berea sandstone cores showed additional oil recovery ranging from 4 to 17% OOIP through injection of low-salinity brine (5000 ppm NaCl) as a secondary recovery mode. As the permeability increased from 6 to 167 md, an additional oil recovery up to 32.9% of OOIP was observed by low-salinity waterflooding. None of the three sandstone rock types (Buff Berea, Grey Berea, and Parker) showed a response in the tertiary recovery mode. A significant incremental oil recovery of 6.9% OOIP was recovered in the tertiary mode for the Bandera sandstone rock. No direct relation was found between the total clay contents and oil recovery. In addition to the clay content, the sandstone rock quality and minerals distribution appears to play a key role in the effectiveness of low-salinity waterflooding. The rock quality has a significant effect in the performance of low-salinity waterflooding. The incremental oil recovery increased from 4 to 17% when the average pore-throat radius (R35) of the core increased from 1.3 to 8.7 microns. Introduction Waterflooding is the most common type of supplementary recovery in which water is injected into the reservoir and displaces oil towards the producing zone. In the conventional waterflooding, the used injection water may be taken from the nearest available source. These sources include produced water, rivers, lakes, seawater, and aquifers. Historically, the physical mechanism behind this improvement in oil recovery was attributed to the pressure maintenance and displacement of oil by injected water. Based on the conventional view, the injection brine composition and salinity were believed to have no effect on the efficiency of oil recovery by waterflooding (Schumacher 1978). Hughes and Pfister (1947) pointed out that brines would keep the clay content of producing sands in a permanently flocculated condition, and therefore, brines were recommended for use in the secondary recovery mode by waterflooding. Over the last decade several laboratory studies and field tests have shown that low-salinity waterflooding (LSW) and smart waterflooding improved oil recovery compared to high-salinity waterflooding (HSW) for sandstone and carbonate reservoirs. LSW flooding involves injecting brine with a lower salt content or ionic strength. Previous laboratory and field tests indicated that the injected brine was in the range of 500–5,000 parts per million (ppm) of total dissolved solids (TDS) (Yildiz and Morrow 1996; Nasralla and Nasr-El-Din 2011). Yildiz and Morrow (1996) showed that changes in injection-brine composition can improve recovery, thereby introducing the idea that the composition of brine could be varied to optimize waterflood recovery. Tang and Morrow (1997) noticed that LSW has a good potential to improve oil recovery. Tang and Morrow (1999) concluded that the presence of clays, initial water saturation, and crude oil were all necessary for LSW to increase oil recovery.